The leaders of South Korea and Vietnam agreed Friday to work together to increase bilateral trade to more than US$100 billion by 2020.

South Korean President Moon Jae-in and Vietnamese President Tran Dai Quang also agreed to deepen diplomatic ties and enhance development cooperation during their summit in Hanoi. Moon arrived in the Vietnamese capital Thursday.

"I am very glad to have visited Vietnam again as the destination of my first overseas trip this year. Vietnam is a very special country to South Korea and a key partner in the New South Policy that our government is actively pursuing," President Moon said at the start of his second summit with Quang, according to Cheong Wa Dae pool reports.

The two leaders met on the sidelines of the Asia Pacific Economic Cooperation summit in Danang, Vietnam, in November. Moon's first state visit to Hanoi is part of his two-nation trip that will later take him to the United Arab Emirates.

Quang welcomed his South Korean counterpart, noting his visit will not only benefit both people of their countries but also contribute to peace, stability and prosperity in their shared region and the world.

Moon took the opportunity to offer an apparent apology on behalf of his nation for South Korea's participation in the Vietnam War.

"While we continue to develop our such exemplary cooperative relationship, I express my regrets for the unfortunate history between the two countries that remains in our minds and hope the two countries will join their efforts to increase their future-oriented cooperation," he told the Vietnamese leader.

South Korean President Moon Jae-in (L) and Vietnamese President Tran Dai Quang shake hands before the start of their bilateral summit in Hanoi on March 23, 2018. (Yonhap)

In a joint declaration released shortly after their summit, the two leaders noted each other's country has become one of the most important partners.

"The leaders of the two countries highly evaluated the fact that the countries have greatly improved their relations over the past 25 years since the South Korea-Vietnam diplomatic ties were established," read the joint declaration at a joint press conference.

"In addition, the leaders agreed on the need to upgrade the South Korea-Vietnam strategic cooperative partnership by expanding and deepening their exchange in each area while solidifying their complementary, active and creative cooperation by continuing their achievement in the development of bilateral ties over the past 25 years under the spirit of mutual respect," they said in their declaration.

Efforts to enhance their ties must include increased exchanges between their governments and people in all areas.

"The two leaders agreed to establish a strong foundation for the development of the countries' strategic cooperative partnership by promoting mutual exchanges in various areas, such as central and local governments, the parliament, political parties and various social organizations," the joint declaration said.

Moon and Quang also agreed to increase their countries' diplomatic and defense cooperation.

The countries will launch regular dialogue between their foreign ministers, while their defense ministries will seek to announce a joint statement on defense cooperation and shared vision at an early date, they said.

The countries will also work to boost their exchanges in the education, cultural, tourism and welfare sectors, they added.

All these efforts will help the countries realize their goal of boosting their bilateral trade to over $100 billion by 2020.

"The two leaders agreed to increase balanced and sustainable trade to achieve the goal of $100 billion in annual trade by 2020," the joint statement said.

In 2017, the countries' bilateral trade came to $63.9 billion, Cheong Wa Dae said earlier. Vietnam is South Korea's fourth-largest trading partner, and South Korea is Vietnam's second largest.

The leaders have also agreed to make joint efforts to further promote free trade in their region by accelerating negotiations for the Regional Comprehensive Economic Partnership, a multilateral free trade agreement that currently involves 16 nations, including China and all 10 member states of the Association of Southeast Asian Nations (ASEAN).

Moon has pledged to visit all 10 ASEAN countries, including Vietnam, before his five-year term ends in 2022.

Vietnamese President Tran Dai Quang (L, in black suit) walks with South Korean President Moon Jae-in to inspect the Vietnamese honor guard in a ceremony held in Hanoi on March 23, 2018, to welcome the South Korean leader, who arrived the previous day on a three-day state visit. (Yonhap)

Friday's summit also led to the conclusion of negotiations for a social security agreement, under which some 13,000 South Korean business people and laborers currently in Vietnam will be exempt from Vietnam's new mandatory social welfare program over the next five years, according to Cheong Wa Dae.

President Moon explained his upcoming summit with North Korean leader Kim Jong-un, asking for Vietnam's support for the rare meeting that he has said may help denuclearize the Korean Peninsula.

President Quang welcomed the recent rapprochement between the two Koreas and said he hoped to see the peaceful resolution of the North Korean nuclear issue, according to Cheong Wa Dae.

Earlier in the day, Moon visited the mausoleum of Ho Chi Minh to pay his respects to the late former leader of Vietnam.

He is scheduled to attend a state dinner hosted by President Quang later in the day.

Moon is also scheduled to meet three other top Vietnamese leaders, including Nguyen Phu Trong, general secretary of the Communist Party, and Prime Minister Nguyen Xuan Phuc.